The prior art is replete with various types of paging systems and radio operated systems by which a message can be left for an individual who is not able to be contacted directly for one reason or another. For example, many paging systems operate with a large complex central processing facility in which messages are queued and transmitted, normally in digital form, to subscribers along with the subscriber's address code. The subscriber to the service carries a paging unit which is pre-programmed to activate upon receiving a message which is preceded by the address code for that paging unit. The pager then normally emits an audible sound to alert the subscriber that a message is being received and stored for him. The message is placed in the pager memory and the message is retrieved in the form of a display message, normally on an LED or LCD display screen. Although such systems are efficient and require very little air time in order to send the digital message, the messages transmitted are necessarily of limited duration and are normally of the type which require the subscriber to go to the nearest telephone and call the message originator. In addition, unless the transmitted messages are strictly numeric, i.e. telephone numbers and the like, alpha numeric messages require a special terminal in order to input the alpha numeric message to be transmitted and paging systems of this type require expensive computerized central message facilities.
Other paging systems are available which utilize a transmitter which transmits in analog form an audio message preceded by an address code which is received by a pre-programmed receiver. The message is played immediately upon receipt and in some units the message can be recorded on a tape cassette for replay. Pagers of this type are normally relatively bulky and require substantially high power requirements to drive the mechanical portions of the tape recorder.
In the area of telephone communications, answering machines are available which are provided with one or more tape cassettes for playing a message to the caller to indicate that the called party is not available to answer the phone and to record a message for later playback. Although answering machines are readily available for single line use and their price is becoming more and more reasonable, such devices are normally not available as part of the telephone circuitry itself and most of the existing answering machines are bulky and require a substantial amount of desk top space. Furthermore, answering machines are not readily available for multi-line business phones and com-line recording.
Yet another form of message service is the so called voice storage retrieval system (VMS) in which a voice message can be left at a central message storage facility and the subscriber, by use of a specific code, can access the memory at the central computer to retrieve the message. These systems are expensive to operate in view of the necessity of powerful computers at the central system facility to process and store the messages and in addition can be inconvenient to use since the subscriber must find a telephone in order to receive the message. In addition, messages may not be timely received because the addressee inadvertently fails to check for messages.
In the area of two-way radio communication, such as in the case of police and fire communications, emergency communications and the like where the addressee may be away from the mobile unit from time to time, many systems employ the use of hand held receivers, i.e. walkie-talkies, which may be patched into the mobile receiver for the receipt of incoming messages while the operator is away from the unit. Such devices are expensive and in many cases would be totally unnecessary if a reliable, inexpensive message storage system were available at the mobile unit. Some systems are available which are similar to the telephone answering machines for transmitting a prerecorded message and for recording incoming messages when the operator is not at the mobile unit. These systems have been found to be bulky, unreliable and inflexible in connection with radio communications. A more sophisticated system has been promulgated in U.S. Pat. No. 4,468,813 Burke et al, and in U.S. Pat. No. 4,495,647 Burke et al. This system requires a base unit which sends a command program packet in digital form to the mobile unit which is programmed to respond to the command program for receiving the message in analog form. Responsive to the command program, the mobile unit converts the message to digital form for storage and responsive to a termination command sent in digital form by the base unit, the mobile unit recording system is deactivated. The operator at the mobile unit can then replay the digital message in analog form. The system as disclosed in the aforementioned U.S. Patents requires a sophisticated encoding system at the base transmitter which is capable of generating a command program packet and the termination code signal. Furthermore, the base transmitter must be capable of trnsmitting the command packet in the form described in the aforementioned patents. The mobile unit must be capable of receiving and decoding the command program packet and transmitting its own command program packet back to the base unit. The mobile unit utilized in such a system requires two separate power supplies which would render the device unsuitable for portable hand held receivers such as pagers and the like.